Rubber goods such as tire treads often are made from elastomeric compositions that contain one or more reinforcing materials such as, for example, particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. Reduced hysteresis and traction are, to a great extent, competing considerations: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa.
Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of these properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of fillers can be improved by increasing their interaction with the elastomer(s), which commonly results in reductions in hysteresis (see above). Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer, typically at a terminus thereof.
Various natural and synthetic elastomeric materials often are used in the manufacture of vulcanizates such as, e.g., tire components. Some of the most commonly employed synthetic materials include high-cis polybutadiene, often made by processes employing catalysts, and substantially random styrene/butadiene interpolymers, often made by processes employing free radical or anionic initiators.
Of particular difficulty to synthesize are interpolymers of olefins and polyenes, particularly conjugated dienes, due in large part to the very different reactivities of those two types of ethylenically unsaturated monomers. Their respective susceptibilities to coordinate with the metal atoms of polymerization catalysts differ greatly.
Although difficult to synthesize, such interpolymers are of significant commercial interest. Because polyene and olefin monomers usually originate from different raw materials and are provided via different techniques, manufacturers of elastomeric materials can guard against supply and price disruptions of either monomer by synthesizing interpolymers with varying and/or adjustable amounts of mer from each.
Additionally, certain portions of pneumatic tires, particularly sidewalls, preferably exhibit good resistance to atmospheric degradation, particularly ozone degradation. Such components can benefit from inclusion of substantially saturated elastomer(s). Historically, typical options have included ethylene/propylene/non-conjugated diene (EPDM) interpolymers or brominated copolymers of isobutylene and para-methylstyrene. Alternatives to these materials also remain desirable.